1. Field of the Invention
The present invention is related to broadband communication systems that use Data Over Cable Service Interface Specification (DOCSIS) media access protocol or any of its derivatives. Particularly, the present invention applies to the reuse of Synchronous Code Division Multiple Access (S-CDMA) related hardware (e.g., timestamp, minislot and frame count hardware) to create an extended mode to DOCSIS 2.0, namely to allow the Time Division Multiple Access (TDMA) channel to have any minislot size as is afforded to the S-CDMA channel.
2. Related Art
Cable modems provide communications on cable networks. In general, a user connects a cable modem to the TV outlet for his or her cable TV, and the cable TV operator connects a cable modem termination system (“CMTS”) in the operator's headend. The CMTS is a central device for connecting the cable network to a data network like the Internet. The CMTS is a central distribution point for a cable system. Data flows “downstream” from the CMTS to the cable modem (i.e., downstream channel). Alternatively, data flows “upstream” from the cable modem to the CMTS (i.e., upstream channel).
A common cable modem standard today is the Data Over Cable Service Interface Specification (“DOCSIS”). DOCSIS defines technical specifications for both cable modems and CMTS.
DOCSIS 2.0 introduces a multiple-access scheme via the concept of multiple logical channels per physical channel. The primary motivation for this is to allow modems of different types to coexist on the same physical spectrum, including TDMA (Time Division Multiple Access) and S-CDMA (Synchronous Code Division Multiple Access). S-CDMA and TDMA require different sets of channel parameters and their physical layer transmissions are not compatible. Therefore, the multiple logical channel approach is necessary to allow these modems to share the same physical spectrum on the cable plant.
To coordinate upstream data transmission on multiple logical channels from multiple modems to the CMTS, the modems send request messages to the CMTS. These request messages indicate to the CMTS the amount of bandwidth needed to transmit the data. The bandwidth expressed in minislots is sometimes referred to as the physical length of the data to be transmitted. The physical length required to transmit a data packet having a given byte length varies depending upon the overhead imposed by the physical layer (e.g., TDMA or S-CDMA) of the cable system. The parameters that determine the overhead may include, but are not limited to, the preamble, guard band, forward error correction, and padding.
In DOCSIS 2.0, an upstream channel is defined by a UCD (upstream channel descriptor), which is a type of message broadcast from the CMTS to all modems. UCD parameters may include, but are not limited to, center frequency, symbol rate, physical layer type (e.g., S-CDMA or TDMA), burst-specification items such as modulation order and forward error correction (FEC) codeword size, etc. When a request to transmit data is made by a modem, the byte length of the data packet to be transmitted is converted to the physical length by calculations based on a formula that includes the UCD parameters.
Upstream bandwidth is divided into minislots, which are the smallest time unit utilized for bandwidth requests and grants. When a grant message is returned to the requesting modem, it tells the requesting modem which minislots to use for upstream transmission of the data packet. Because the minislot is the unit in which upstream capacity is allocated to modems, one parameter the modem needs to operate on a channel is the size of the minislot. Minislot size differs between TDMA and S-CDMA channels, which is explained next.
On TDMA channels, minislot size is limited to power-of-two sizes and is given explicitly in a dedicated field in the UCD message. Here, the minislot size field is characterized by its size T, defined in units of timebase ticks of 6.25 μs. Allowable values for T are T=2M when M is one of 1 through 7. That makes T=2, 4, 8, 16, 32, 64 or 128. Thus, 1 tick equals 6.25 μs which equals 64 counts of the timestamp reference clock. The modem uses this value from the UCD in conjunction with the symbol rate to determine how many symbols are in one minislot.
TDMA assigns different time slots to different modems (or users). During the time slot assigned to one modem, all other modems remain silent and therefore there is no interference between the modems.
One reason to limit minislot size to power-of-two sizes means that the system timestamp count and the minislot count for a particular channel always have a fixed relationship. Here the minislot count is derived by a binary shift-right by a given number of bits of the system timestamp count.
S-CDMA differs from TDMA in that minislot size is not limited to power-of-two sizes. With S-CDMA, the incoming data is organized in minislots which have two dimensions including spreading codes and time. The time duration of minislots is one S-CDMA frame that spans a programmable number of S-CDMA symbol intervals. Generally, the maximum frame length is 32 S-CDMA symbol intervals. Symbol spreading is performed through multiplication by a spreading code (spreading sequence) of 128 chips taken from a set of 128 orthogonal codes that are generated by a quasi-cyclic shift.
In DOCSIS 2.0, an S-CDMA spreading interval is equal to 128 TDMA symbol intervals. Two ways to control minislot size with S-CDMA are to control the size of the frame and/or to control the number of codes in a minislot.
Since minislot size in S-CDMA is not limited to power-of-two sizes, this means that the system timestamp count and the minislot count for a particular channel do not have a fixed relationship. S-CDMA uses a timestamp snapshot constructed to periodically indicate the relationship between these two quantities and also between them and the frame counter.
When minislot size can be more finely controlled to accommodate data packets, the result is a more efficient use of bandwidth. For example, with TDMA the minislot size is limited to one of 2, 4, 8, 16, 32, 64 or 128 ticks. Assuming that a 17 tick per minislot is needed to hold a data packet, in TDMA a 32 tick minislot would be used. This wastes 15 ticks of bandwidth. Therefore, what is needed is a way of allowing the TDMA channel to have the same flexibility with minislot size as is afforded to the S-CDMA channel without the burden (e.g., complexity, cost, and schedule) of additional hardware to perform a separate set of calculations.